Keyboard eavesdropping

Funny thing. A technique for eavesdropping keyboards based on the sound they produce has been published:

Each key on computer keyboards, telephones and even ATM machines makes a unique sound as each key is depressed and released, according to a paper entitled “Keyboard Acoustic Emanations” presented Monday by IBM research scientist Dmitri Asonov. All that is needed is about $200 worth of microphones and sound processing and PC neural networking software. Today’s keyboard, telephone keypads, ATM machines and even door locks have a rubber membrane underneath the keys. “This membrane acts like a drum, and each key hits the drum in a different location and produces a unique frequency or sound that the neural networking software can decipher,” said Asonov.